The present invention relates to a toner in which a medium-resistance external additive is added to control the amount of charge and an image forming apparatus.
FIG. 1 is an illustration for explaining a conventional image forming apparatus with an intermediate transfer member. A rotary developing unit 1 comprises developing rollers of four colors: Y, M, C, K. The developing unit 1 turns to bring every one of the developing rollers in contact with the photoreceptor 2 for every rotation of a photoreceptor 2, just like a revolver. After the photoreceptor 2 is uniformly charged by a charging unit 3, images for the respective colors are exposed to light so as to form electrostatic latent images on the photoreceptor 2. The photoreceptor 2 comes in contact with the developing rollers Y, M, C, K to develop respective color images. Because of contact between the photoreceptor 2 and an intermediate transfer member 5, these color images are transferred to the intermediate transfer member 5 (primary transfer). Residual toner particles remaining on the photoreceptor after the transfer are removed by a cleaning unit 4. As the four color images are superposed on each other on the intermediate transfer member 5, a paper 8 is conveyed and a secondary transfer roller 7 is brought in contact with the paper 8. At this point, secondary transfer bias voltage is applied by a power source (not shown) so as to transfer the superposed color images to the paper 8 at once (secondary transfer). The secondary transfer roller 7 can swing to come in contact with the intermediate transfer member 5 in the direction of the double arrow. While color images are primary transferred and toned on the intermediate transfer member 5, the secondary transfer roller 7 is spaced apart form the intermediate transfer member 5. During the secondary transfer, the secondary transfer roller 7 is in contact with the intermediate transfer member 5 via the paper. Residual toner particles remaining on the intermediate transfer member 5 after the secondary transfer are removed by a cleaning unit 6.
Toners to be used in the image forming apparatus having the aforementioned structure have particle size distribution. The range of the particle size distribution depends on manufacturing conditions of the toner, so sometimes narrow (particles are regular in size) and sometimes broad (particles are irregular in size, i.e. there are particles of various sizes). In general, in case of a toner prepared by the pulverization method, the particle size distribution should be relatively broad. Classification can help to somewhat narrow the range of the particle size distribution. However, there is a limit from the technical point of view and increase in cost is inevitable. In case of a toner prepared by the polymerization method, the particle size distribution should be relatively narrow, but all particles can never be of the same size.
When such a toner having s particle size distribution is used, a phenomenon, called xe2x80x9cselective consumption developmentxe2x80x9d, in which toner particles in a certain particle size range are consumed prior to other toner particles occurs. That is, in a developing unit 10 shown in FIG. 2, toner particles are conveyed from a toner container 11xe2x86x92a supply roller 12xe2x86x92a developing roller 13 in this order, and then charged to be developed. During this, the toner is regulated by a regulating blade 14 into a predetermined thickness. In case of a non-magnetic toner, the supply roller 12 rotates in a direction opposite to the rotational direction of the developing roller 13 and is in contact with the developing roller so as to rub the toner onto the surface of the developing roller to cause frictional electrification. In this manner, toner particles of the non-magnetic toner are supplied from the supply roller 12 to the developing roller 13. When there is a small amount of toner particles on the developing roller 13, for example, in case of supplying toner particles to the developing roller 13 for the first time or immediately after a solid image is developed, toner particles in the toner container 11 are supplied to the developing roller 13 without any selection. On the other hand, when there is a somewhat larger amount of toner particles on the developing roller 13, for example, after a white solid image or characters or lines with low density is developed, toner particles with diameters which are easily caught by the developing roller 13 stay on the developing roller 13 and toner particles with diameters which are hardly caught by the developing roller 13 come off the developing roller 13 and are thus collected in the developing unit, depending on the roughness of the developing roller, the conditions of regulation, and the conditions of supply.
Since most of outputs of image forming apparatuses are low density images such as characters and lines just like the latter case, such toner particles with diameters which are easily caught by the developing roller are consumed prior to the other toner particles. Toner particles to be easily caught by the developing roller depend on the conditions such as the roughness of the developing roller. Originally, these conditions are set to facilitate that toner particles with diameters equal or close to the mean particle diameter exhibit their performance. Therefore, toner particles with diameters equal or close to the mean particle diameter or in a range about the roughness of the developing roller are consumed prior to the other toner particles. This phenomenon is called xe2x80x9cselective consumption developmentxe2x80x9d.
FIG. 3 is a graph showing a particle size distribution of toner particles which remain in the developing unit due to the selective consumption development. In this graph, the original particle size distribution is indicated by D. Toner particles with diameters to be consumed due to the selective consumption development are the most abundant particles. When the residual quantity of the toner is reduced by repeated consumption, the ratio of toner particles of other diameters smaller or larger than the mean particle diameter or than the roughness of the developing roller is increased. As toner is refilled, toner particles with diameters easily consumed are consumed prior to the other toner particles so as to increase the concentration of the other toner particles, resulting in a distribution indicated by Dxe2x80x2.
When the quantity of toner particles not suitable for developing is increased in the developing unit because the residual quantity of toner is reduced and the toner is repeatedly refilled, toner particles with small diameters and large diameters which are not suitable for developing must be consumed for developing. In this case, since the amount of charge on toner particles with small diameters is high relative to the mass thereof, these are hardly developed because of the mirror-image force of the charge so as to stay on the developing roller. Therefore, the small toner particles may be damaged by friction of the regulating blade and the like so that these are easily adhered to the developing roller and the like, thereby leading to the occurrence of filming. On the other hand, since the amount of charge on toner particles with large diameters is low relative to the mass thereof, these are easily conveyed by developing filed and thus easily developed. However, in case of using large toner particles, flush or interruption may be produced in the transfer process, thus degrading a resultant image.
To prevent the aforementioned problems due to the selective consumption development, there is a method of agitating toner particles in the developing unit. However, the selective consumption of toner particles is caused by outputs of images and thus dependent on image patterns, Due to repeat printing with insufficient agitation, undesired concentration of toner particles is partially caused. By fully agitating the toner particles to provide a uniform concentration, the progress of undesired concentration can be slowed. However, this method is just slowing the concentration due to the selective consumption, and is not a radical measure.
There is another method of peeling toner particles from the developing roller during every rotation. For example, when the supply roller is in contact with the developing roller with increased pressure, the toner particles are peeled from the developing roller during every rotation. Therefore, the state on the developing roller should be just like the state after a solid image is outputted so that the toner particles are uniformly consumed regardless of the particle size. Since this method leads to large torque-up and toner damage, however, preventative measures are necessary for practical use of this method.
The present invention is made in order to solve the aforementioned problems. In the present invention, the medium-resistance external additive coating ratio of toner particles of which charge is to be leaked is changed according to the diameter of toner particles, thereby making the charge-to-mass ratio constant and preventing the occurrence of selective consumption development.
Therefore, the first aspect of the present invention is a toner being characterized in that the medium-resistance external additive coating ratio of toner particles of which mother particles have equivalent particle diameters smaller than an equivalent particle diameter of a mother particle diameter equal to the mean particle diameter of the toner is set to be higher than a virtual reference curve in synchronous distribution of the equivalent particle diameters of synchronous medium-resistance external additive particles relative to the equivalent particle diameters of mother particles, wherein assuming that the medium-resistance external additive coating ratio of a toner particle of which a mother particle has an equivalent particle diameter equal to the roughness of a developing roller or the mean particle diameter of the toner is a reference value, the virtual reference curve is obtained to satisfy that the medium-resistance external additive coating ratio is constant at the reference value.
The second aspect of the present invention is a toner being characterized in that the medium-resistance external additive coating ratios of toner particles in a range in which mother particles have equivalent particle diameters smaller than d1 and in a range in which mother particles have equivalent particle diameters larger than d2 are set to be higher than a virtual reference curve in synchronous distribution of the equivalent particle diameters of synchronous medium-resistance external additive particles relative to the equivalent particle diameters of mother particles, wherein assuming that the medium-resistance external additive coating ratio of a toner particle with a diameter between d1 and d2 is a reference value wherein d1 is an equivalent particle diameter of a mother particle equal to the roughness of the developing roller and d2 is an equivalent particle diameter of a mother particle equal to the mean particle diameter of the toner (d1 less than d2), the virtual reference curve is obtained to satisfy that the medium-resistance external additive coating ratio is constant at the reference value.
The third aspect of the present invention is a toner being characterized in that the medium-resistance external additive coating ratio of toner particles in a range in which mother particles have equivalent particle diameters larger than the roughness of the developing roller or than an equivalent particle diameter of a mother particle equal to the mean particle diameter of the toner is set to be higher than a virtual reference curve in synchronous distribution of the equivalent particle diameters of synchronous medium-resistance external additive particles relative to the equivalent particle diameters of mother particles, wherein assuming that the medium-resistance external additive coating ratio of a toner particle of which a mother particle has an equivalent particle diameter equal to the roughness of a developing roller or the mean particle diameter of the toner is a reference value, the virtual reference curve is obtained to satisfy that the medium-resistance external additive coating ratio is constant at the reference value.
The fourth aspect of the present invention is an image forming apparatus comprising a photoreceptor on which an electrostatic latent image is formed, a developing unit for developing the electrostatic latent image on the photoreceptor with a toner, a transfer means for transferring the developed image on the photoreceptor, and a fusing means for fusing the transferred image, the image forming apparatus being characterized that said toner is a toner of any one of the aforementioned aspects.